(351b) Density Measurement of Molten Fluoride Salts

The measurement of molten fluoride salt density is important for the design of Fluoride Salt - Cooled High Temperature Reactors (FHRs). Density plays a crucial role in neutronics analysis, and it is essential to consider the temperature dependence of density in natural convection analysis for FHRs. Density measurements of two molten fluoride salt mixtures were performed at the University of Wisconsin (UW)-Madison: LiF-BeF₂ (FLiBe) and LiF-NaF-KF (FLiNaK) at temperatures near melting (459 ÌŠ and 454 ÌŠ respectively) to 800 ÌŠ C. Density measurement experiments took place inside an Argon glovebox in order to protect laboratory personnel from beryllium hazards and to maintain water and oxygen levels below 4 ppm to avoid salt contamination. A Mettler Toledo analytical balance XS104 was used for measurements. The scale comes equipped with a small hook underneath the top loading tray from which a wire is hanged. The opposite end of the wire is hooked to a nickel bobber which is then submerged in the molten salt. By using Archimedes principle and the known volumetric expansion of the nickel bobber, the density of the molten salt mixture is obtained from the buoyant weight measurement shown on the scale. Extensive calibrations for the experiment apparatus were performed for temperature and mass readings to reduce the amount of measurement uncertainty. The experimental data was then compared to reported literature values, as well as to a prediction of density using additive molar volumes of the constituents. Preliminary data collected for FLiBe shows agreement within 6% of density values from NIST measurements with a 0.2% uncertainty. Results for FLiNaK failed to yield a linear fit at temperatures near melting, from 456 to 525 ÌŠ C, which contributed to an uncertainty of 2.7%. Work is currently underway to produce repeatable results for FLiBe measurements and to remeasure FLiNaK density. Changes in the experimental apparatus and methodology will involve a verification of isothermal conditions in the salt sample during measurement and characterization of samples pre and post measurement.